The structure of the surface of a sample can be observed by irradiating the sample with an electron while scanning the electron, and detecting a secondary charged particle emitted from the sample. This is called a scanning electron microscope (hereinafter abbreviated as “SEM”). On the other hand, the structure of the surface of a sample can be observed also by irradiating the sample with an ion beam while scanning the ion beam, and detecting a secondary charged particle emitted from the sample. This is called a scanning ion microscope (hereinafter abbreviated as “SIM”). In particular, when a sample is irradiated with a light mass ion species such as hydrogen or helium, a sputtering action is relatively decreased, and therefore, it is preferred for observation of the sample.
As the ion source for an ion microscope, a gas field ionization ion source is preferred. The gas field ionization ion source is an ion source to be used as an ion beam by ionizing a gas by an electric field formed by an emitter tip. The ion source includes a gas ionization chamber having a needle-shaped emitter tip capable of applying a high voltage therein, and to the gas ionization chamber, an ionized gas is supplied from a gas source through a gas supply pipe. When the ionized gas (or a gas molecule) supplied from the gas supply pipe approaches the apex of the needle-shaped emitter tip to which a high voltage is applied so that a high electric field is applied, an electron in the gas (gas molecule) tunnels through the potential barrier lowered by the electric field, and therefore is converted to a positive ion and emitted. This is utilized as an ion beam. The gas field ionization ion source can generate an ion beam with a narrow energy width. Further, since the size of the ion generation source is small, a fine ion beam can be generated.
In order to observe a sample with a high signal-to-noise ratio by an ion microscope, it is necessary to obtain an ion beam with a high current density on the sample. Due to this, it is necessary to increase the ion radiation angle current density of a field ionization ion source. In order to increase the ion radiation angle current density, the molecular density of an ionized gas (ion material gas) in the vicinity of an emitter tip may be increased. A gas molecular density per unit pressure is inversely proportional to the temperature of a gas. Therefore, the temperature of the gas around the emitter tip may be decreased by cooling the emitter tip to an extremely low temperature. By doing this, the molecular density of the ionized gas in the vicinity of the emitter tip can be increased. It is also possible to increase the molecular density of the ionized gas in the vicinity of the emitter tip by increasing the pressure of the ionized gas in the vicinity of the emitter tip. For example, the pressure of the ionized gas around the emitter tip is from about 10−2 to 10 Pa.
PTL 1 discloses a method in which platinum is vacuum deposited onto the apex of a tungsten emitter tip, and then, platinum atoms are moved to the apex of the emitter tip under high temperature heating, whereby a pyramid structure in the order of nanometers (which is determined to be called “nanopyramid”) of platinum atoms is formed, a method in which the nanopyramid is formed by field evaporating the emitter tip in vacuum, and a method in which the nanopyramid is formed by ion beam irradiation.